A lot of people have noticed the purple spots on Michael Phelps and some of the other Olympic athletes. Did they get in a fight with an octopus? Nope, they were suction cupping!
To give a very brief overview: suction cupping is an ancient Chinese therapy method where glass, bamboo, or plastic jars are placed on bare skin to create a vacuum which helps with blood circulation. It is a great way to heal damaged tissue or increase muscle relaxation. The darker the spots, the more tissue damage.
This is a great article on it.
We have been using suction cupping for years at my house. It dramatically helped my brother with his asthma. It is really cool to see Olympic athletes utilizing the same treatment type! While the circles may look strange, they really help with healing.